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MS ABI: Consider alignment attributes on typedefs for layout

Authored by majnemer on Jul 29 2014, 3:45 PM.



The MS ABI has a notion of 'required alignment' for fields; this
alignment supercedes pragma pack directives.

MSVC takes into account alignment attributes on typedefs when
determining whether or not a field has a certain required alignment.

Do the same in clang by tracking whether or not we saw such an attribute
when calculating the type's bitwidth and alignment.

This fixes PR20418.

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majnemer updated this revision to Diff 11999.Jul 29 2014, 3:45 PM
majnemer retitled this revision from to MS ABI: Consider alignment attributes on typedefs for layout.
majnemer updated this object.
majnemer added a reviewer: rnk.
majnemer added a subscriber: Unknown Object (MLST).
rnk edited edge metadata.Jul 29 2014, 4:00 PM

I'm skeptical that only one bit is enough information here...

147 ↗(On Diff #11999)

I'd rather just throw this in clang::. We can use it in record layout.

1658–1663 ↗(On Diff #11999)

Grumble grumble these helpers are just asking you to do multiple hash table lookups.... Nothing to do here, though.

675 ↗(On Diff #11999)

Can you also add this test, just to satisfy my curiosity?

typedef int __declspec(align(2)) my_int_t;
#pragma pack(push, 1)
struct A {
  char a;
  my_int_t b;
#pragma pack(pop)
static_assert(sizeof(A) == 6, "");

I suspect because the required alignment is actually *lower* than the natural alignment of int, we will take the max alignment between the alignment attribute and the natural alignment get sizeof(A) == 8.

majnemer updated this revision to Diff 12004.Jul 29 2014, 6:04 PM
majnemer edited edge metadata.
  • Address review feedback
rnk accepted this revision.Jul 29 2014, 6:17 PM
rnk edited edge metadata.


2264–2265 ↗(On Diff #12004)

Add a comment that this ignores alignment attributes, i.e. it gets the natural alignment.

This revision is now accepted and ready to land.Jul 29 2014, 6:17 PM
majnemer closed this revision.Jul 29 2014, 6:39 PM
majnemer updated this revision to Diff 12006.

Closed by commit rL214274 (authored by @majnemer).

emaste added a subscriber: emaste.Jul 30 2014, 12:16 PM
emaste added inline comments.

This will need a corresponding change to LLDB